When the brain or spinal cord is injured, damaged neurons are unable to regenerate their axons and the functions carried out by these neurons are lost to the individual. A major challenge in neuroscience is to understand the factors responsible for the failure of axon regeneration and to design treatments and therapeutic strategies that allow axon growth. Traumatic injury is almost always accompanied by the formation of a glial scar at the injury site. This complex tissue is thought to be a barrier to axon regeneration due to the expression of specific growth-inhibitory molecules. Among these molecules are the chondroitin sulfate proteoglycans. One of the growth-inhibitory proteoglycans found at the glial scar is the NG2 chondroitin sulfate proteoglycan. Immunoaffinity-purified NG2, recombinant fragments of NG2, and cell membranes containing NG2 inhibit axon growth and repel and collapse growth cones in vitro. Levels of NG2 increase rapidly after injury and reach a peak at about 7 days after injury, a time when damaged axons begin to try to regenerate. The long-term goal of this proposal is to understand the basis of the growth-inhibitory functions of NG2 at sites of CNS injury and to design experimental treatments that can reverse this growth inhibition. Our specific aims are to 1) use loss-of-function approaches to determine whether blockade or removal of NG2 from newly forming glial scars allows axon regeneration in vivo. We will apply neutralizing antibodies against NG2 to the injured spinal cord and crushed optic nerve and then assess the effects on axon regeneration. We will also measure axon regrowth after spinal cord injury in a new strain of NG2 null mice. Our second aim is to identify the regions within domains 1 and 3 of the NG2 core protein responsible for axon growth inhibition. This work will focus on the functions of the laminin G domains of NG2 and of a newly recognized proteoglycan repeat element. In the third aim, we will use gain-of-function approaches to determine whether the expression of NG2 can render cell surfaces non-permissive for axon growth during development and after peripheral nerve injury. This approach will allow us to test the role of secreted NG2 and of the chondroitin sulfate glycosaminoglycan chains in axon inhibition. The results to be obtained here will lay a strong basic scientific foundation for clinical treatments designed to improve the usually disastrous outcome of spinal cord injury.